1. Field of the Invention
The present invention relates to a hydrogen occluding alloy which exhibits high electric discharge characteristics (high output characteristics) in practical use for a battery cathode, in particular.
2. Discussion of the Background
Conventionally, for a battery cathode, for example, a hydrogen occluding alloy has been used in which the characteristics of the hydrogen absorption and desorption rates and the initial activation of the battery have been improved by action of a rare earth element hydride which is dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure. One such system based on a Ni based hydrogen occluding alloy is described in Japanese Patent Laid-Open No. 25528/1998, which has a composition comprising, by weight percent hereinafter "%" indicates "weight %"),
(i) 32-38% of rare earth elements essentially consisting of La and /or Ce, PA1 (ii) 0.1-17% of Co, PA1 (iii) 0.5-3.5% of Al, PA1 (iv) 0.5-10% of Mn, PA1 (v) 0.005-0.5% of hydrogen and PA1 the balance Ni and the unavoidable impurities. The alloy has a microstructure of a rare earth element hydride dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure in a ratio of 0.5-20% by area. PA1 (i) 32-38% of at least one of La or Ce, PA1 (ii) 0.1-17% Co, PA1 (iii) 0.1-3.5% Al, PA1 (iv) 0.5-10% Mn, PA1 (v) 0.005-0.1% of hydrogen, with the balance being Ni and unavoidable impurities, wherein said alloy has a microstructure of a phase having a Ce.sub.2 Ni.sub.7 -type crystal structure and rare earth element hydride dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure and, wherein the amount of said phase having a Ce.sub.2 Ni.sub.7 -type crystal structure is 1-40% by area and the amount of said rare earth element hydride is 0.5-20% by area. When the alloy is put to practical use as a battery cathode, it becomes possible for a battery to exhibit high electric discharge characteristics (high output characteristics).
In addition, the hydrogen occluding alloy described above is typically made by preparing a molten alloy having a given composition and casting it into an ingot. When placed into practical use as a battery cathode, for example, the ingot is subjected to temper annealing in a vacuum or nonoxidizing inert gas atmosphere at a given temperature ranging from 900 and 1050.degree. C. for a given time period, if necessary. Moreover, the as-cast or temper-annealed ingot is subjected to hydrogenation heat treatment in a hydrogen atmosphere at given temperature ranging from 600-950.degree. C. for given time period to form rare earth element hydride and so that a microstructure is produced in which the rare earth element is dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure. Subsequently, the hydrogenated ingot is mechanically pulverized to a predetermined particle size or pulverized in a process which employs a pressurized hydrogen atmosphere which includes hydrogen absorption at a given temperature within the range from 10-200.degree. C. and hydrogen desorption by vacuum evacuation.
In addition, when the hydrogen occluding alloy is used in, for example, a battery cathode, the battery can be practically used after an initial activation treatment in a pressurized hydrogen atmosphere for a given time period until the cathode including the hydrogen occluding alloy has a sufficient discharge capacity at an initial stage of use.
On the other hand, recent attempts have been made in a number of experiments to apply a hydrogen occluding alloy to a battery cathode which is required to have a larger output such that it can be used in electric tools, electrically assisted bicycles and electric automobiles. However, the hydrogen occluding alloy above and other alloys can not be practically used now as a battery cathode, since a sufficient output (high electric discharge characteristics) of the battery containing the cathode can not be obtained.
In view of the above problems, the present inventors have attempted to develop a hydrogen occluding alloy for use in the fabrication of a battery cathode, which exhibits a high output, thereby making it possible for practical use in battery construction. A hydrogen occluding alloy which has high hydrogen absorption and desorption rates and initial activation characteristics is particularly desired. As a result, the following has been found: When the conventional hydrogen occluding alloy of the as-cast or temper-annealed ingot described above is subjected to hydrogenation heat treatment to form a rare earth element hydride. If the alloy is heated from room temperature to a given temperature ranging from 200-400.degree. C. in vacuum or inert gas atmosphere and subsequently the heat treatment is done at a temperature from 400-1000.degree. C. for a given time period in a hydrogen atmosphere and cooled, the rare earth element hydride is formed which has a microstructure of a phase having a Ce.sub.2 Ni.sub.7 -type crystal structure dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure. As a result, after the heat treatment above, the alloy has a microstructure of a phase having a Ce.sub.2 Ni.sub.7 -type crystal structure and rare earth element hydride dispersively distributed in a matrix having a CaCu.sub.5 -type crystal structure. In addition, when the alloy above, having a microstructure in which the amount of a phase having a Ce.sub.2 Ni.sub.7 -type crystal structure is 1-40% by area and the amount of a rare earth element hydride is 0.5-20% by area, which is formed by controlling the condition of heating and hydrogenation heat treatment above, was used as a battery cathode, the output of the battery is very high because of the existence of the Ce.sub.2 Ni.sub.7 -type crystal structure. Moreover, high rates of hydrogen absorption and desorption are obtained, as well as good initial activation characteristics, both of which characteristics are significantly promoted by the presence of the rare earth element hydride.